The problem I have is that i want to start and stop the timer between 2 points and use momentary switches at each end of the track, these would replace S2 (in parallel?) on the diagram. The idea is that when a car passes the switch and makes the contact it starts/stops the timer.

So how would i change the single on/off switch for 2 momentary switches.

The timer reset is already a momentary switch so that is not a problem.

I would like to add a few LEDs to be on or off depending on wether a car has activated the timer, but need to sort the switch first. any help is really appreciated

I would like to place a microswitch at each end of the pitlane to start and stop the timer. These momentary switches can be activated by the car guide blade pressing on the switch arm and making the contact as it passes over.

I would use a third microswitch to control the reset function as I would like to keep the time displayed until another car makes entry to the pit lane.

Because there is one switch that controls the timer on/off by being open and closed I need to keep the circuit "on" when a car presses switch one until it passes switch 2 and turns timer off. I presume the easiest way to do this is with flip flops?

I was looking at flip flops on wiki, I have a little knowledge and can build a circuit from a schematic, but thats about all. From what I can see would an RS flip flop be what I rerquire, and would i need one for each switch?

Could you please help a little more and help to show how it would be included in the schematic.

Mechanical switches are slow, the contacts bounce, and they will affect the speed of your car.

If you want it to be a decent design, you really need to use something like an opto-interruptor. These are much like optoisolators/optocouplers, except they have a slot between the IR transmitter and the IR receiver, so that an object passing between the two will change the state of the output.

You could use one opto interruptor to trigger the SET input of a f/f, and another to trigger the RESET input of a f/f. Have the opto interruptor mounted under the track so that the guide pin passes through the slot.

Thanks for the reply, I am open to options but would need for someone to show how this would be incorporated into a schematic. How would the switch stay in a closed posistion until the car passes sensor 2 and leaves the timed area, as the original schematic uses 1 switch to operate the timer.

If you want at least SOME degree of accuracy, you'd have to start with a crystal oscillator. For better accuracy, you could start with an oven-controlled crystal oscillator (OCXO).
Roman Black posted a how-to on making your own OCXO:http://www.romanblack.com/xoven.htm
Of course, it takes awhile for the ocxo to reach a stable temperature.

A crystal is just the start. Then you need a divide-by circuit to get the frequency down to the range where you can count the fractions of seconds.

The reason I have decided to use this schematic using the 555 is because I have no programming knowledge and would be even further out my depth that I already am.

Are there any alternatives that dont involve going any more complex, that a total beginner could get to grips with. I looked online for a kit based stopwatch but could not find any with 7 segment display.

Could I use a latching relay to keep a momentary switch closed to replace S2 and put a second switch in parallell to operate it from the other end of the track to start and stop the clock? I know it wont be very accurate and i would like to keep it very simple.

OK so getting back to switches, what would be the simplest way of converting to a momentary switch?

Click to expand...

On the more direct side a standard NO / NC / COM microswitch, with a lever and possibly a roller attached, could be triggered as you thought, by the car passing the guide blade....... sorry about the overkill otherwise